As is disclosed in, for example, U.S. Pat. No. 4,103,566 and JP,A(U)(Japanese Utility Model Publication under Art. 13bis of the Japanese Utility Model Law) No. 61-135047, a transmission of the type set forth above is, in general, fashioned such that a key-shift transmission mechanism is disposed in a power transmission path relatively at an upstream portion where a relatively low torque is transmitted. For heightening vehicle-driving force, the speed-change shaft of key-shift transmission mechanism is connected to wheel axles through a reduction gearing. On an outer surface of transmission casing is disposed a brake for braking the vehicle which brake is associated to the speed-change shaft or to a speed-reduction shaft for the reduction gearing.
For example, the transmission disclosed in U.S. Pat. No. 4,103,566 is fashioned such that, while a drive shaft is located between a speed-change shaft and co-axially disposed wheel axles, two sets of speed-reduction gear trains are interposed between the speed-change shaft and wheel axles by utilizing larger and smaller two gears rotatably mounted on the drive shaft. One end of the speed-change shaft extends outwardly from the transmission casing and has thereon a co-rotatable rotor which is braked for a vehicle-braking purpose. In the transmission disclosed in JP, A(U) No. 61-135047 referred to above, speed-reduction shaft is additionally interposed between a speed-change shaft and wheel axles and one and another speed-reduction gear trains are disposed between the speed-change shaft and speed-reduction shaft and between the speed-reduction shaft and wheel axles. One end of the speed-reduction shaft extends outwardly from the transmission casing and carries thereon a brake drum which is adapted to be braked by means of internal brake shoes.
U.S. Pat. No. 4,662,241 discloses a transmission including a key-shift transmission mechanism which has a very compact structure adapted for use in a small-sized working vehicle. As shown in FIGS. 1 and 2 of this U.S. patent, a set of drive gears are mounted rotatably on one of the left and right wheel axles and are driven to rotate by an input bevel gear, having an integral spur gear, which is mounted rotatably on a speed-change shaft. A set of speed-change gears meshing respectively with the drive gears are mounted rotatably on the speed-change shaft and are coupled to the change shaft one at a time using a shift key for a speed-change purpose. The speed-change shaft is connected to the wheel axles through a speed-reduction gear train. No brake is provided.
In each of the transmissions referred to above, a differential gearing is disposed between left and right wheel axles for a smooth turn of vehicle. A final gear of the speed-reduction gearing is used as an input gear of such differential gearing. Speed-change shaft is formed of a solid shaft and is journalled at both ends thereof in a transmission casing.
Although a set of speed-change gears for a key-shift transmission mechanism are disposed on a speed-change shaft such that they are almost in contact with one another, a relatively lengthy shaft is employed as the speed-change shaft. This is because such speed-change shaft further mounts a slidable shifter sleeve which is displaced axially of the change shaft for shifting a gear-coupling shift key by a distance corresponding to an axial width occupied by the speed-change gears. A solid speed-change shaft according to the prior art is disadvantageous with respect to material cost of a transmission. Machining for forming an elongated key-receiving axial groove in the outer surface of a lengthy change shaft will highten the cost. Such lengthy, solid shaft is also disadvantageous with respect to weight of the transmission.
The prior art transmission including a brake which is disposed on an outer surface of transmission casing so as to brake a transmission shaft projected outwardly of the casing requires a brake case on the transmission casing and also a prolonged shaft. Accordingly, such transmission is disadvantageous with respect to manufacturing cost, compactness and weight of the transmission.
As compared to the structure disclosed in U.S. Pat. No. 4,662,241, referred to above, which includes only one reduction gear train between a speed-change shaft and wheel axles, the structure known from U.S. Pat. No. 4,103,566 and JP, A (U) No. 61-135047, referred to above, which includes two sets of reduction gear trains is advantageous in that speed change is attained at a lower torque portion and then vehicle-driving torque may be enlarged without any trouble. However, the former structure reduces the size of a transmission casing in a direction across plural transmission shafts disposed in parallel within the transmission casing, whereas the last-mentioned structure increases number of transmission shafts disposed in parallel within a transmission casing and, therefore, increases the size of transmission casing in a direction across such transmission shafts.